Many modern vehicles include a relatively-low-cost range sensor, such as an ultra-wide band short-range radar. Range sensors are generally orientated in a longitudinal direction in the front and/or rear of a vehicle, and satisfactorily determine a distance from the sensor to objects, such as other vehicles. Despite this reasonable performance for longitudinal sensing, range sensors have limited resolution in lateral measurements because of their relatively small aperture size. Thus, while helpful for determining a distance by which the host vehicle is behind another vehicle or other object ahead in a same lane, they are not as helpful for gauging kinematical characteristics of a vehicle that is in an adjoining lane.
Also, because range sensors typically track only one or two points per object, they often confuse objects that are close to each other, such as two vehicles crossing as they switch lanes.
Many modern vehicles include a vision system including at least one camera. In one example, a monocular camera has a good amount of pixels (e.g., 640 pixels) spread across a relatively-small field-of-view (e.g., 50 degrees). The camera is often directed toward a blind-spot area, such as adjacent or behind the host vehicle, for providing visual information to a driver. Such cameras, though, are not relied on for accurate measurements of longitudinal range. Efforts have been made to estimate range to an object by using a camera to detect a bottom part of the object adjacent the ground and assuming that the ground is always flat. This approach, though, does not reliably yield accurate results because the ground is of course rarely actually flat due to factors such as water-drainage gradients, inclines, declines, varying vehicle-pitch angle, occlusion of objects, and super-elevations (e.g., highway turn banks).